1. Field of the Invention
This invention relates to a fuel control apparatus, and more particularly to a fuel control apparatus which can remarkably improve the combustion of lean fuel mixtures in internal combustion engines.
2. Description of the Related Art
A conventional method of controlling the air-fuel ratio in a fuel supply system of a gasoline burning type internal combustion engine, is described in Japanese Patent Laid-Open Publication No. 58-41231. As described therein, the air-fuel ratio is increased to reduce fuel cost in a low load operational range, and feedback control is adopted in a mid load operational range in which the air-fuel ratio is maintained at the so-called theoretical value, in order to purify the exhaust gas. In the heavy load operational range, the air-fuel ratio is made smaller than the theoretical air-fuel ratio to maintain the developed power. If the above described control method is achieved perfectly, problems of fuel economy, purification of exhaust gas, drivability and the like can theoretically be solved.
In setting such an air-fuel ratio, however, errors are produced depending upon a change with time of the accuracy of various sensors and actuators, and the fuel controlling apparatus cannot actually employ the set values described above with respect to each load. Further, the air-fuel ratio has to be set at a considerably high value for the purpose of fuel economy, and burning or ignition must continue to be satisfactory even with a lean air-fuel mixture.
To this end a scheme for improving ignition has recently been studied. This scheme divides an intake port portion into two parts and employs a valve for generating vortexes in one part thereof which is to be closed during partial load so as to increase the airflow rate in the intake port portion and utilizes a swirl flow for the air-fuel mixture in the combustion chamber so that the fuel is easily burned.
This scheme is advantageous in that its structure is simple and that, since, it acts in correspondence with the negative pressure during intake, the degree of opening can be set approximately by the load. On the other hand, the scheme is disadvantageous in that the throttling effect produced during low load is not sufficient to provide a swirling flow of the air-fuel mixture in the combustion chamber. Furthermore, during full load, draft resistance is inevitable in spite of the full opening of the valve.
In addition, although it is considered that the swirl flow produced in the combustion chamber is effective for atomization of fuel and promotion of combustion, it is difficult to maintain the swirl flow during each intake and compression stroke under every operational condition.
It has been also proposed to properly select fuel injection timing and to perform feedback control of the air-fuel ratio by using an air-fuel ratio sensor which estimates the actual air-fuel ratio from the concentration of residual oxygen in the exhaust gas. In this case, though combustion of a lean mixture is accomplished in a normal partial load, performance at the time of starting, during the transitional period and at a maximum output of full opening of the throttle valve, is, at most, the same as that of a conventional engine.